The present invention relates to a method of producing a metallic shell for a spark plug of an engine.
Japanese Patent Application Unexamined Publication No. H7-16693 shows a spark plug including a metallic shell. Referring now to FIG. 4 and FIG. 5, the spark plug and the metallic shell are explained. As shown in FIG. 4, a spark plug 1 includes a generally tubular metallic shell 3 having a through hole 2 that extends through the metallic shell 3 in an axial direction of the metallic shell 3. An insulator 4 is mounted into the through hole 2 of the metallic shell 3. An electrode mounting hole 5 extends through the insulator 4 in a direction of a central axis of the insulator 4. A center electrode 6 is disposed in a tip end-side portion of the electrode mounting hole 5. A ground electrode 7 is arranged such that one end portion thereof is fixedly attached to an axial tip end face of the metallic shell 3 and the other end portion thereof is opposed to a tip end face of the center electrode 6.
The metallic shell 3 includes a screw shaft portion 10 to be screwed into a spark plug mounting tap hole 9 of an engine (i.e., a cylinder head) 8, and a flange-shaped stop portion 11 which is disposed on a rear side (i.e., one axial end side) of the screw shaft portion 10 (that is, on an upper side thereof as shown in FIG. 4 and FIG. 5) and larger in outer diameter than the spark plug mounting tap hole 9. The metallic shell 3 further includes a tool engagement portion 12 that is disposed on a rear side (i.e., one axial end side) of a stop portion 11 and engageable with a tool. The tool engagement portion 12 is formed into a shape, for instance, a hexagonal prism shape, suitable for engagement with the tool such as a wrench. A caulking portion 13 which holds the insulator 4 to the metallic shell 3 is disposed at a rear end of the tool engagement portion 12. The caulking portion 13 is formed in such a manner that a tubular portion extending in an axial direction of the tool engagement portion 12 as shown in FIG. 5, is rounded by caulking as shown in FIG. 4. A grooved portion 14 is disposed between the stop portion 11 and the tool engagement portion 12.
The above conventional art discloses a method of producing the metallic shell 3 of the spark plug 1 having the above-described structure. The method is now explained by referring to FIGS. 3A1-3F and FIGS. 6A1-6C. As shown in FIG. 3A1 and FIG. 6A1, a cylindrical wire rod made of a suitable material, for instance, a low carbon steel, is cut to a predetermined length to thereby prepare a metal blank M. Next, the metal blank M is subjected to upsetting by using a cold forging machine (not shown) and thereby formed into a bullet-shaped workpiece N having a round chamfered portion 15 as shown in FIG. 3A2 and FIG. 6A2. The round chamfered portion 15 extends over an entire circumference of a tip end of the workpiece N.
Next, a first die 180 shown in FIGS. 6B1 and 6B2 is prepared. The first die 180 includes a first large diameter cavity 180a larger in diameter than the workpiece N, a first small diameter cavity 180b smaller in diameter than the workpiece N, and a tapered cavity 180c between the first large diameter cavity 180a and the first small diameter cavity 180b. The first large diameter cavity 180a is configured to form a portion of the workpiece N which is later formed into the stop portion 11 of the completed metallic shell 3. The first small diameter cavity 180b is configured to form a portion of the workpiece N which is later formed into screw shaft portion 10 of the completed metallic shell 3. The tapered cavity 180c is connected to a terminal end of the first large diameter cavity 180a and an initial end of the first small diameter cavity 180b. 
Subsequently, as shown in FIG. 6B1, the workpiece N is inserted in the first large diameter cavity 180a of the first die 180. As shown in FIG. 6B2, the workpiece N is extruded so as to conform to the first large diameter cavity 180a, the tapered cavity 180c and the first small diameter cavity 180b, while being pressed by punch 190. The workpiece N is thus formed into a first intermediate article N1 including a large diameter portion Na and a tapered portion Nc which are formed into a base of the stop portion 11 of the completed metallic shell 3, and a small diameter portion Nb that is formed into a base of the screw shaft portion 10 of the completed metallic shell 3. The first intermediate article N1 shown in FIG. 3B is thus obtained.
Next, as shown in FIG. 6C, a second die 200 is prepared. The second die 200 has a stepped cavity including a second large diameter cavity 200a larger in diameter than the large diameter portion Na of the first intermediate article N1, and a second small diameter cavity 200b into which the small diameter portion Nb of the first intermediate article N1 is insertable. The second large diameter cavity 200a and the second small diameter cavity 200b are continuously arranged to form the stepped cavity.
Subsequently, the first intermediate article N1 having the second large diameter portion Na, the tapered portion Nc and the small diameter portion Nb is inserted in the stepped cavity of the second die 200 which includes the second large diameter cavity 200a and the second small diameter cavity 200b. The first intermediate article N1 is then pressed by a punch 210 and extruded to thereby form a second intermediate article N2. Upon the extrusion, a hole Nd later serving as the through hole 2 of the completed metallic shell 3 is formed by the punch 210, and at the same time, the large diameter portion Na and the tapered portion Nc are expanded in a radial direction thereof so as to conform to the second large diameter cavity 200a. The second intermediate article N2 shown in FIG. 3C is thus obtained.
Next, the second intermediate article N2 is subjected to extrusion with a cold forging machine and thereby formed into a third intermediate article N3 shown in FIG. 3D. The third intermediate article N3 is then subjected to punching with a cold forging machine and thereby formed into a fourth intermediate article N4 shown in FIG. 3E.
Next, the fourth intermediate article N4 is subjected to extrusion with a cold forging machine and thereby formed into a fifth intermediate article N5 shown in FIG. 3F. The fifth intermediate article N5 is then subjected to rolling to form a male-thread portion on an outer circumferential surface of the small diameter portion Nb which corresponds to the screw shaft portion 10 of the completed metallic shell 3. The fifth intermediate article N5 is then subjected to machining or cutting to form a grooved portion that corresponds to the grooved portion 14 between the tool engagement portion 12 and the stop portion 11 of the completed metallic shell 3. Thus, the completed metallic shell 3 shown in FIG. 4 is obtained.